This invention relates to a portable radio communication device and, in particular to a wristwatch-type radio communication device having an improved shielding structure for effectively shielding against high frequency noise transmitted to the analog circuitry for receiving radio signals and/or transmitting radio signals.
A variety of portable radio receivers and transmitters such as electronic wristwatches having an FM radio set, a wristwatch-type paging device and the like are currently being manufactured and commercialized. The wristband portion of the device has been utilized to accommodate certain electronic components. For example, in a conventional wristwatch-type radio receiver, the wristband is integrally provided with electrically conductive plates or films to form a loop antenna, when worn on the user's wrist as disclosed in U.S. Pat. No. 4,713,808 issued to Gaskill, European Patent No. 339482 and Japanese Patent Nos. 1046325 and 63252002. These devices disclose wristwatch-type radio receivers with a wristband antenna and the disclosures thereof are incorporated herein by reference as if fully set out herein. However, such wristwatch-type devices have restricted space for mounting electronic components and elements.
Generally, the wristwatch-type radio receiver constructed with a wristband antenna includes a power source battery, signal receiving circuitry and signal processing circuitry accommodated in the housing of the receiver. The signal processing circuitry utilizes digital signals applied to a decoder, a memory and the like. Further, the digital signal processing circuitry is controlled by driving pulse signals. The driving pulse signals are formed with high frequency components. The high frequency components create internal or external noise causing disturbance to the radio signals received by the signal receiving circuitry. Accordingly, the increased noise degrades the signal receiving sensitivity of the radio receiver. The internal noise due to the high frequency components reaches the receiving circuitry directly through the signal lines of the device. On the other hand, the external noise is transmitted to the outside of the device from the digital signal processing circuitry and then received by the receiving circuitry through the wristband antenna.
The prior art has proposed solutions to reduce the internal noise by providing filter elements in the signal lines. In this manner, signals having a certain range of frequency or noise components are not transmitted through the filter elements. In addition, the prior art has proposed an alternative design having a component layout wherein the signal receiving circuitry is separated from the signal processing circuitry in the plane direction of the housing by the maximum distance. This construction was designed to minimize the adverse effect of the noise created by the signal processing circuit. Further, the external noise can be eliminated in the device by enclosing either the signal receiving circuit or the digital circuit (i.e, the noise source) in a shielding box.
However, the above structures of shielding noise are not satisfactory for the wristwatch-type receiving and/or transmitting devices. Since the wristwatch-type device is provided with restricted space for accommodating components, the additional provision of filter elements or shielding boxes makes component layout complicated or near impossible. Further, such approaches increase the cost and size of the device. More specifically, the device is formed too large in the plane direction. This increased size also arises where the signal receiving circuitry is positioned away from the signal processing circuitry in the housing.
It is, therefore, desirable to provide a wristwatch-type receiver having an effective noise shielding structure without increasing the size or the manufacturing cost of the device.